The present invention relates to a preparation process of partial glycerides making good use of a glycerolysis reaction of oil or fat by a lipase.
Preparation processes of partial glycerides include an esterification process and a glycerolysis process, and each process includes cases making use of a chemical catalyst and cases making use of an enzyme catalyst. As an esterification process making use of an enzyme catalyst, there has been known a process in which partial glycerides are prepared from oleic acid or the like high in fatty acid purity and glycerol as described in Japanese Patent Publication No. 56311/1994 and Japanese Patent Application Laid-Open No. 19090/1987. However, this process is not preferable as the preparation process of the partial glycerides from the economical point of view because a fatty acid prepared by decomposing oil or fat at a high temperature and a high pressure and then removing a colored component by a distilling treatment is used, and glycerol obtained by dehydration, concentration of purification treatments of an about 10% aqueous solution of glycerol produced upon the decomposition of the oil or fat is used. Accordingly, the glycerolysis process in which both fatty acid group and glycerol group constituting the partial glycerides are derived from a cheap oil or fat, and a deficient glycerol group can be supplied as glycerol is effective as the industrial production process of the partial glycerides.
The processes for preparing the partial glycerides from such a cheap oil or fat and glycerol by an enzymatic glycerolysis reaction are reported in Japanese Patent Publication No. 12710/1992, JAOCS, 71, 3, 339 (1994), etc.
However, the process described in Japanese Patent Publication No. 12710/1992 requires a special lipase satisfying conditions under which no secondary production of free fatty acids in a low water content region is attendant, so that it is difficult to preset a water content in a reaction system and procure the lipase. On the other hand, in the process described in JAOCS, 71, 3, 339 (1994), the partial glycerides are prepared from hydrogenated hardened beef tallow and glycerol by a glycerolysis process by which the formation of fatty acids is inhibited. However, this process has involved a problem that reproducibility of the reaction is poor.
Accordingly, it is an object of the present invention to provide a process for preparing a partial glyceride with industrial advantage by an enzymatic glycerolysis reaction.
The present inventors have found that when in a glycerolysis reaction of oil or fat making use of an easily available lipase, the reaction is conducted in a system in which water is present, under conditions that the secondary production of fatty acids occurs to a great extent and that crystals are precipitated in the reaction system, which are entirely different from the usual common sense, the yield of a partial glyceride, particularly a diglyceride is enhanced with great strides, thus leading to completion of the present invention.
According to the present invention, there is thus provided a process for preparing a partial glyceride, which comprises, in a glycerolysis reaction of oil or fat making use of a lipase, conducting the reaction in the presence of water under conditions that crystals are partially precipitated in the reaction system in the course of the reaction and the concentration of free fatty acids in an oil phase amounts to at least 5% by weight.
According to the present invention, there is also provided a process for preparing a diglyceride, which comprises conducting a removing operation of fatty acid, glycerol and monoglyceride components from a reaction mixture in the reaction described above or after completion of the reaction.
In the present invention, oil or fat, a lipase, glycerol and water are used as raw materials. Examples of the oil or fat include vegetable oils and animal oils, more specifically, soybean oil, rapeseed oil, cotton-seed oil, corn oil, rice oil and fish oil. Oil or fat, in which a proportion of saturated fatty acids in the acyl group is at most 30% by weight, is preferably used. When the content of the saturated fatty acids is high, not only a high temperature is required of the reaction, but also a reaction product cannot be taken out of a vessel after the reaction, or heating is required, and so handling is complicated because the melting point of the raw oil or fat itself is high and the melting point of a partial glyceride formed is higher. In addition, a problem of deactivation of the enzyme arises in the high-temperature and high-water content system.
No particular limitation is imposed on the lipase used. However, an enzyme (1,3-position-selective lipasexe2x80x9d) specifically acting on the 1- and 3-positions of glycerol is preferred. More preferably, lipases derived from microorganisms of the genera Rhizopus, Aspergillus and Mucor, and splenic lipases, more specifically, lipases derived from Rhizopus delemar, Rhizopus japonicus, Rhizopus niveus, Aspergillus niger, Mucor javanicus and Mucor miehei may be used. Immobilized lipases obtained by immobilizing each of the lipases on various kinds of carriers (Celite, diatomaceous earth, silica gel, ion-exchange resin, etc.) may also be used. In general, the use of the latter lipases, i.e., (heat-resistant) immobilized lipases having high durability in a wide concentration range is preferred to the use of the former lipases. Many of these lipases or immobilized lipases are easily available as products on the market. The amount of the lipase used is preferably 0.1 to 30% by weight, particularly 1 to 15% by weight (200 to 100,000 units per g of oil or fat) based on the raw materials for the reaction.
No particular limitation is imposed on the water content in the process according to the present invention. However, it is preferably 5 to 50% by weight, particularly 8 to 30% by weight, in terms of an initial concentration in the reaction system, based on glycerol from the viewpoint of acceleration of the reaction. With the progress of the reaction, the water content in the reaction system somewhat varies. However, the reaction is scarcely affected thereby.
Fatty acids and partial glycerides may exist in the raw materials for the reaction in addition to the oil or fat, glycerol and water. However, the number of moles of glycerol group is preferably controlled to a range of 0.3 to 3, particularly 0.8 to 1.5 per mole of the fatty acid group in the whole raw material.
The reaction conditions in the present invention require that crystals are partially precipitated in the reaction system in the course of the reaction and the concentration of free fatty acids in an oil phase amounts to at least 5% by weight. The conditions under which crystals are partially precipitated in the reaction system in the course of the reaction are conditions that oil or fat to be reacted is naturally melted in the initial stage of the reaction, but parts of monoglycerides and free fatty acids formed with the progress of the reaction are precipitated.
Thus, it is assumed that both of monoglycerides and free acids are partly excluded from the phase system of the reaction when brought in the condition under which a part of crystals is precipitated in the course of the reaction, whereby the glycerolysis reaction is further accelerated. Such conditions vary according to the oil or fat used as a raw material. When oil or fat, in which a proportion of saturated fatty acids in the acyl group is at most 30% by weight, is used as the oil or fat, the conditions are preferably achieved by presetting the reaction temperature to a range of 0 to 250xc2x0 C., particularly 0 to 150xc2x0 C.
In the present invention, the conditions are preset in such a manner that the concentration of free fatty acids in an oil phase amounts to at least 5% by weight, preferably 8 to 30% by weight, the yield of the partial glycerides is enhanced with great strides. Such conditions are preferably achieved by presetting the reaction temperature to a range of 0 to 25xc2x0 C., particularly 0 to 15xc2x0 C. like the conditions for the precipitation of crystals.
The fact that the yield of the partial glycerides is enhanced by the presetting of these reaction conditions is contrary to the idea of the conventional processes described in Japanese Patent Publication No. 12710/1992 and the like that conditions that the water content is low and no free fatty acid is formed are preset. This is considered to be attributable to the fact that the conventional view of glycerolysis reaction mechanism have been wrong. More specifically, the glycerolysis reaction of oil or fat (TG) with glycerol have generally been considered to be as follows. Namely, a fatty acid group in the oil or fat is bonded to glycerol (GLY) to form monoglycerides (MGs) and diglycerides (DGs) as shown in the following scheme. The water content in the reaction system at this time is considered to be a factor that the enzymatic activity (reaction rate) is changed. Namely, the reaction has been considered to be an ester exchange reaction or alcoholic group exchange reaction in which no fatty acid is formed. 
However, when an experiment of the glycerolysis reaction is conducted, hydrolysis occurs even in a region of a trace water content to form fatty acids. Even when the rate equations of the respective components are calculated out from the above formula to conduct kinetic analysis, the presence of the fatty acids formed cannot be theoretically proven. The reaction of a lipase has generally been considered to take place at an interface between an oil phase and a water phase. However, this cannot be explained since the reaction occurs even in a nonaqueous system containing no water.
Therefore, the reaction mechanism of the glycerolysis in the case where a 1,3-position-selective lipase is used as an enzyme catalyst is considered to be as follows. Namely, no alcohol group (ester) exchange reaction takes place, and hydrolysis, ester synthesis, conversion into MG and conversion into DG only take place. A reaction site is considered to be a contact point between the oil phase and the enzyme, and the concentrations of the respective components in the rate equation are regarded as concentrations in the oil phase. More specifically, it has been considered that the concentrations of the fatty acids and glycerides constituting the oil phase, and the concentration of dissolved water and GLY contribute to the reaction, while water and GLY potion in the aqueous solution phase of glycerol do not contribute to the reaction and only affect the concentrations of water and GLY in the oil phase. When experimental data were subjected to fitting (numerical analysis) using simultaneous differential equations of the rate equation on such hypothesis, the calculated result consisted with the experimental date with considerable accuracy. When the experimental results at various temperature in a range in which no crystal was precipitated were also subjected to numerical analysis to calculate out respective rate constants. These values were Arrhenius-plotted against the temperatures. As a result, linearity is obtained. Therefore, the validity of the model was able to be proven, and so it is considered that the glycerolysis reaction progresses in accordance with the following scheme. 
More specifically, the mechanism that partial glycerides is formed from oil or fat and glycerol using an enzyme as a catalyst is considered to be as follows. Namely, TG is hydrolyzed to form DGs and fatty acids, the fatty acids and GLY are subjected to ester synthesis to form MGs, and further esterification occurs to synthesize DGs. Such elucidation of the mechanism of the glycerolysis permits proving that the yield of the partial glycerides is enhanced.
As described above, the reaction temperature is preferably suitably adjusted according to the kind of the resin used at a temperature range of 0 to 25xc2x0 C. (particularly 0 to 15xc2x0 C.), and the reaction time is suitably 10 to 200 hours, particularly 20 to 100 hours.
The reaction may be further continued after removing the crystals precipitated in the course of the reaction outside the system. However, the crystals may be removed after completion of the reaction. In order to collect the partial glycerides from the reaction mixture, it is preferable to conduct the above crystal-removing operation, water phase- (GLY phase-) removing operation and distilling operation in suitable combination.
In order to obtain diglycerides in the process according to the present invention, it is only necessary to conduct the removing operation of the fatty acid, glycerol and monoglyceride components from a reaction mixture. Parts of the fatty acid and monoglyceride components can be removed by the crystal-removing operation. Water, glycerol and lipase can be removed by the water phase-removing operation. The fatty acids, monoglycerides and glycerol in the oil phase can be efficiently removed by distillation, particularly molecular distillation. Accordingly, these operations are combined, whereby the intended product (diglyceride concentration: at least 55% by weight, triglyceride concentration: at most 30% by weight) high in diglyceride content can be obtained.